Bolt Gap Comparison

[W.E.G.]

I'm counting myself lucky that the bolt that came with my Century CETME gaps at 0.019"

Today I took the opportunity to compare several other bolt/carrier assemblies to the original assembly from my rifle, just to see how the specs worked out.

Here is the result

http://i227.photobucket.com/albums/dd7/rkba2da/rifle%20pics/CETME/BoltGapComparisonchart.jpg

http://i227.photobucket.com/albums/dd7/rkba2da/rifle%20pics/CETME/BoltGapComparisonpic.jpg

[okie shooter]

You do have a few variables there, the bolt carriers will vary slightly and the fact did you shoot the rifle to "settle" the parts in. That said, its interesting information.

[bolex]

So reading your data that means that the best bolt when compared to the new indicates that it has 0.004" wear.

[W.E.G.]

...did you shoot the rifle to "settle" the parts in.

That has occurred to me.

I want to use the new bolt/carrier for a build. I'm pondering whether I should shoot 30-40 rounds with it in my Century CETME in order to "settle" the parts to ensure that the builder achieves optimum placement when pressing the barrel.

I guess I could just cycle the bolt manually. But, letting that bolt fly without a round in the magazine causes the cocking handle to fly forward so violently, I'm always afraid its gonna collapse the cocking tube from the impact of the handle.

[rustypirate]

The only thing I don't see in your comparison is the roller size.

I am going to assume that all of the rollers are the same size, then your test shows a variation of wear on the bolt head and locking piece.

It is interesting to note that the New-In-Wrap bolt assembly measures out over the recomended spec, kind of like they expected it to "break-in" and loose a few thousandths.

[W.E.G.]

I guess that would have to assume that the bolt assembly in my Century rifle was new when assembled. I can't vouch for that.

Assuming my Century bolt assembly was used before Century installed it, and further assuming the barrel was pressed to ideal bolt-gap, we can conclude that my barrel may be pressed just a smidgen deeper than if the gun had been assembled from all-new parts.

I don't know the roller sizes on all those bolt assemblies. However, I suspect that the bolts would show more wear, if they had been used for so long that roller replacement became necessary. I'd bet than none of the bolts have oversize rollers.

The only bolt that looks like seriously tired iron is the first one I pulled out of the pile, and which remarkably turned out to be the one that was ground. What are the chances of that?!!!

[tomoshenko]

I guess I could just cycle the bolt manually. But, letting that bolt fly without a round in the magazine causes the cocking handle to fly forward so violently, I'm always afraid its gonna collapse the cocking tube from the impact of the handle.

I assume you have appropriate gap between carrier and cocking piece?

[loner42]

I don't see what the problem is here. All you are showing is the variation in Wear of the bolt, LP, and rollers of 8 bolt carrier assemblies and only 2 are out of tolerance. Were they all measured in the gun with the gun assembled, boltcarrier dropped from full retract and slammed home and hammer dropped? Any of them would work and be safe in your gun except the 2 and one of them was the ground bolt which you would'nt want to use anyway.

Does anyone think they should ALL measure the same? Also as was mentioned earlier what size are the rollers? You,ve got alot of variables here that don't mean anything except they are all safe to use except 2.

I'd just pick one in the mid range and move on.......................

[Otis61]

That has occurred to me.

I want to use the new bolt/carrier for a build. I'm pondering whether I should shoot 30-40 rounds with it in my Century CETME in order to "settle" the parts to ensure that the builder achieves optimum placement when pressing the barrel.

I guess I could just cycle the bolt manually. But, letting that bolt fly without a round in the magazine causes the cocking handle to fly forward so violently, I'm always afraid its gonna collapse the cocking tube from the impact of the handle.

You could do that, but alot of the wear occurs in the trunion.

[tomoshenko]

I guess that would have to assume that the bolt assembly in my Century rifle was new when assembled. I can't vouch for that.

Assuming my Century bolt assembly was used before Century installed it, and further assuming the barrel was pressed to ideal bolt-gap, we can conclude that my barrel may be pressed just a smidgen deeper than if the gun had been assembled from all-new parts.

I don't know the roller sizes on all those bolt assemblies. However, I suspect that the bolts would show more wear, if they had been used for so long that roller replacement became necessary. I'd bet than none of the bolts have oversize rollers.

The only bolt that looks like seriously tired iron is the first one I pulled out of the pile, and which remarkably turned out to be the one that was ground. What are the chances of that?!!!

If you are trying to maximize the number of rounds before having to repress why dont you press it to the max end of spec with -2 rollers?

http://www.hkspecialist.net/small.htm
HK NEW ROLLERS FOR 308/223/9MM 8.0/8.2/8.4/7.8/7.6 SIZES $13.00 EACH OR $26.00 A PAIR

[W.E.G.]

I assume you have appropriate gap between carrier and cocking piece?

Yes, I do. Its just that the cocking handle slaps the cocking tube so hard when the bolt is allowed to slam under full force, but without a cartridge to slow it any. The cocking tube has a bit of dent from the impacts.





I don't see what the problem is here.

Does anyone think they should ALL measure the same?

I'd just pick one in the mid range and move on.......................

There is no "problem."

...and I don't really wish to "move on."
I'm quite enjoying playing with all the parts, and learning while I play.

I never suggested that I expect used bolt/carrier assemblies to gap the same.
I simply think its interesting to observe the differences.

Out of curiosity, has anybody here measured the rollers from a kit bolt/carrier assembly?
If so, how did they compare in size to new, standard-size rollers?
Has anybody ever found a documented instance of a bolt from a CETME kit actually having oversize rollers, just as it came from the kit?

[loner42]

I miked the rollers in 2 of the kits and 3 century builds and they were all within a few tenths of 8 mm.

[hunter_la5]

I miked the rollers in 2 of the kits and 3 century builds and they were all within a few tenths of 8 mm.

correct me if I'm wrong, but aren't all the different roller sizes "within a few tenths" (or to be more accurate, within a few hundreths) of 8mm?

+4s = 8.04mm
+2s = 8.02mm
regular =8.00mm

so all the rollers being "within a few tenths of 8mm" doesn't really tell you anything

[loner42]

That is exactly what i thought and have been trying to verify.

8MM = .3149 or .314 thousandths and nine tenths

.02MM = .00078 or .0008
.04MM= .0015 or one and a half thousandths

.2 MM = .0078 OR seven thousandths and eight tenths
.4 MM = .0157 OR .015 thousandths and seven tenths

So which is the correct size of + or - rollers .02 or .2 ?

I would think .02 which is a gnats ass in the real world, only measureable
with a calibrated set of good mikes that read in tenths. (NOT CALIPERS)

If it is .2 and not .02 then you could measure them with calipers

When i say tenths i'm referring to ten thousandths of an inch....... take one thousandth and slice it into ten equally thick pieces and you'll have one tenth of an inch or .0001

An average piece of paper is around .003 thick.

Hundredths are a mile.........................

[tomoshenko]

correct me if I'm wrong, but aren't all the different roller sizes "within a few tenths" (or to be more accurate, within a few hundreths) of 8mm?

+4s = 8.04mm
+2s = 8.02mm
regular =8.00mm

so all the rollers being "within a few tenths of 8mm" doesn't really tell you anything

The max spread in INCHES from 7.96 Milimeter (-4 ROLLERS) AND
8.04 Milimeter (+4 ROLLERS) IS 0.00315". That ain't a whole lot and easily missed measuring 8.00 mm standards against +2's or +4's if you are using a crappy plastic caliper.

[willie91]

In decimal measurements what would 8mm and +2 and +4s be?

[loner42]

8mm= .3149
+2 = .3157
+4 = .3164
-2 = .3141
-4 = .3144

[W.E.G.]

Good info.

Confirms what I suspected.

No point in tearing-down the bolts to measure rollers with my present tools.

Almost seems like its most cost-effective, if your gap is meager, to simply install the +4's and then measure the new gap spec. After all, a better gap spec is what you are looking for. Worst-comes-to-worst the +4's don't change anything and you are out the price of a set of rollers rather then the price of a high-end micrometer that can measure accurately in "tenth's."

[tomoshenko]

8mm= .3149
+2 = .3157
+4 = .3164
-2 = .3141
-4 = .3144

loner42,
I come up with these, ...but I've had a couple of Jim Beams..

-4: 7.96 mm = 0.313385
-2: 7.98 mm = 0.314173
std:8.00 mm = 0.314960
+2: 8.02 mm = 0.315748
+4: 8.04 mm = 0.316535

[loner42]

loner42,
I come up with these, ...but I've had a couple of Jim Beams..

-4: 7.96 mm = 0.313385
-2: 7.98 mm = 0.314173
std:8.00 mm = 0.314960
+2: 8.02 mm = 0.315748
+4: 8.04 mm = 0.316535

Do we really want to get into Millionths???? :icon_mrgreen:

I'd say we're close enuff.....................

To convert metric to inch you divide the metric number by 25.4

The caculator i'm using is only reading 4 places :thumbup:

[loner42]

W.E.G. You can get a pretty decent set of stainless 6" dial calipers, and a 0-1" mike that reads in tenths for around 50-60.00 each.

Then you could put your little "yeller" plastic calipers back in the Reloadin Drawer..........................:icon_biggrin:,:ic on_biggrin:,:icon_biggrin:

Just messin with ya man.................:rockon:

[W.E.G.]

I'd stick 'em in my drawers, except the scale don't go up high 'nuff. :afro1:

[loner42]

I'd stick 'em in my drawers, except the scale don't go up high 'nuff. :afro1:

Well, maybe you need some 12 inchers , eh ????? :lol2:

[tomoshenko]

Do we really want to get into Millionths???? :icon_mrgreen:

I'd say we're close enuff.....................

To convert metric to inch you divide the metric number by 25.4

The caculator i'm using is only reading 4 places :thumbup:


It's not millionths, it' the -4 you posted is larger than the -2.

[loner42]

It's not millionths, it' the -4 you posted is larger than the -2.


You are 100% correct, shoulda been .3133..................